Fix broken openhab icon

[nandorsoma]

Description

In the README.md the openhab icon link points to a 404 page. This PR intends to fix that.

Related Issue

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🔧 Bug fix (non-breaking change which fixes an issue)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix

Checklist

• I've written tests (if applicable) for all new methods and classes that I created.
• I've added documentation as necessary so users can easily use and understand this feature/fix.

[cla-bot]

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